X-ray imaging systems are intended to observe the inside of an irradiation target body two-dimensionally or three-dimensionally nondestructively by utilizing the high transmissibility of X-rays with respect to the body. The internal density of a body that is a basic physical quantity is used as contrast for such a system to form images. Methods of detecting the density can be broadly divided into two types. One type is used to obtain the density from a change in X-ray intensity, caused by absorption when the X-rays transmit the target body, and the other type is used to obtain the density from a phase shift caused during the transmission. The former type is called “absorption contrast X-ray imaging”, and the latter type, “phase contrast X-ray imaging”.
The imaging systems using the former detection method, namely, absorption contrast X-ray imaging systems, are each composed mainly of an X-ray source, a target body holder, and a detector. In these systems, the target body positioned using the target body holder is irradiated with the X-rays emitted from the X-ray source, then the intensity of the X-rays transmitting the body is detected by the detector, and images are formed. The configuration of these systems are simple in terms of measuring principles. Therefore, these systems are commonly used in many fields, including medical diagnosis, under the name of roentgen systems for two-dimensional observation, and X-ray CT systems for three-dimensional observation by CT (Computed Tomography). However, since hydrogen, carbon, oxygen, and other light elements are almost transparent to X-rays and essentially do not change X-ray intensity, the above systems are low in sensitivity with respect to the biological soft tissues, organic materials, and other target bodies constructed mainly of light elements. To apply such a system, therefore, it is necessary to perform operations such as using the contrast agents that contain a heavy metal(s), or extending an exposure time.
The imaging systems using the latter detection method, namely, phase contrast X-ray imaging systems, require the means that detects phase shifts, in addition to the above system components. Compared with absorption contrast X-ray imaging systems, however, phase contrast X-ray imaging systems are very high in sensitivity and enable observations of biological soft tissues without contrast agents and without harmful levels of X-ray exposure. These advantages are due to the fact that the phase-shift cross-section are about 1,000 times as great in light elements as the absorption cross-section. Examples of phase-shift detection means include as described in Phys. Today 53 (2000) 23: (1) the methods that use an X-ray interferometer described in Japanese Laid-Open Patent Application Publication Nos. Hei 4-348262 and Hei 10-248833, (2) the methods that use an analyzer crystal to detect diffracted X-ray angles described in International Patent Application Laid-Open Publication No. WO95/05725—Pamphlet and in Japanese Laid-Open Patent Application Publication No. Hei 9-187455, and (3) a method that uses Fresnel diffraction.
Table 1 lists comparison results on the physical quantities detected using each of the above-mentioned methods, and on the respective sensitivities, dynamic ranges, spatial resolution levels, and other features and characteristics.
TABLE 1(2) Method using(3) Method using(1) Method using anan analyzerFresnelX-ray interferometercrystaldiffractionDetectionUsing an X-rayUsing BraggDetecting theprinciplesinterferometer todiffraction of aFresnel fringescause interferencecrystal to detectcaused bybetween a materialthe angle ofFresnelwave and a referenceobject-diffracteddiffractionwave, and then detectX-rays (Using thephase shifts from thecrystal as anresulting interferenceangle analyzer)fringesPhysicalcos p∂p/∂x∇2Pquantity(p: density)detectedRelative⊚◯ΔsensitivityDynamicΔ◯◯rangeSpatial10 micronsSeveral micronsSeveral micronsresolutionOthersUnwrapping required.Sensitivity is inA thirda one-dimensionalgeneration ofdirection only.synchrotronradiation or thelike isrequired.
It can be seen from the above table that the method using an analyzer crystal is best balanced with respect to each item. This method has features in that it, compared with the method using an X-ray interferometer, is relatively simple in system configuration, and in that it, unlike the method that uses Fresnel diffraction, does not require a special light source.
A method using an analyzer crystal, concerned with the present invention, will be described hereunder.
When X-rays transmit an object that causes a phase shift φ, if the phase shift φ is spatially nonuniform, a propagating direction of the X-rays will be bent through an angle of θ by diffraction. The angle θ is given as a function of a spatial differential (dφ/dx) of φ, by expression (1).
      (          Numerical      ⁢                          ⁢      expression      ⁢                          ⁢      1        )                                θ          =                                    λ                              2                ⁢                π                                      ⁢                                          ⅆ                ϕ                                            ⅆ                x                                                                          (          1          )                    
A spatial differential of the phase shift can therefore be calculated by detected θ. In addition, the phase can be calculated by spatially integrating θ.
For the method described in International Patent Application Laid-Open Publication No. WO95/05725—Pamphlet, Bragg diffraction of the monolithic crystal in the shape of a flat plate, called the analyzer crystal, is utilized to detect θ, that is, the spatial differential of the phase shift. Bragg diffraction is a phenomenon in which, when the wavelength of an incident X-ray is defined as “λ”, and the lattice spacing of its diffraction plane as “d”, the incident X-ray will be diffracted by the analyzer crystal only if the incident angle θb of the X-ray satisfies the diffraction condition shown in expression (2) below, within an angle range of several seconds.λ=2d sinθb   (2)(Numerical Expression 2)
Therefore, if θb is set so as to satisfy expression (2) when a deflection angle θ of the X-ray in the propagating direction thereof is equal to 0, intensity I of the X-ray diffracted will depend on θ, become a maximum when θ=0, and decrease as increasing of θ. When θ is several seconds of angle, therefore, intensity I of the X-ray will almost equal 0. This phenomenon is utilized to obtain an image of the spatial differential of the phase shift from a spatial distribution of the X-ray's intensity I (i.e., a diffracted image).
Since diffraction intensity I of X-rays is almost constant at θb±θ, measurement of I is only useful for determining the magnitude of θ and provides no information on the direction thereof. The integral of θ, therefore, cannot be calculated, and the phase shift itself cannot be calculated, either. For these reasons, as described in Japanese Laid-Open Patent Application Publication No. Hei 9-187455, the magnitude and direction of θ are detected from the diffraction intensity I that has been acquired at various angles by rotating the analyzer crystal, and then the phase shift is calculated by integration. After this, sectional images of the object are obtained by computed tomography using the phase shift as contrast in combination with the rotation of the sample.